1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus comprising a substantially cylindrical RF coil in which a substantially cosinusoidal current distribution can be generated across a circular-cylindrical circumference by means of a number of rod conductors, extending axially across the cylindrical surface, in cooperation with a number of capacitances. An RF coil for a magnetic resonance imaging apparatus of this kind is known from U.S. Pat. No. 4,339,718 and is based on the well-established fact that a uniform field distribution in such a coil necessitates a cosinusoidal current distribution across the circumference of the cylindrical coil. Coils of this kind are also referred to as "bird-cage" coils and various constructions are available for such magnetic resonance imaging apparatus.
2. Description of the Prior Art
One embodiment of the bird-cage coil serves to generate and detect an RF field for generating and measuring magnetic resonance signals in a region of a body of a patient. Bird-cage coils of this kind are often used in conjunction with a further coil, for example a surface coil, the bird-cage coil then being used for generating an RF field while the surface coil is used for detecting the magnetic resonance signals generated by means of the RF field. Because the frequency of the RF field and of the magnetic resonance signals is the same, the surface coil will interfere with the bird-cage coil during the generating of the RF field, and the bird-cage coil will interfere with the surface coil during the detection of the magnetic resonance signals.
Therefore, one coil must be detuned or deactivated when the other coil is active, and vice versa, in order to prevent undesirable mutual influencing. It is known that a resonance circuit can be detuned by short-circuiting a capacitance. This principle could also be used in the bird-cage coil. However, such a coil consists of a series connection of coils and capacitances. When a capacitance is short-circuited in only one location, the detuning of the bird-cage coil will be so slight that the uncoupling from the surface coil will be insufficient. An obvious solution to this problem would be the short-circuiting of the capacitances in many locations. However, this solution is comparatively complex from a technical point of view and also leads to an undesirable deterioration of the coil quality.